1. Field of the Invention
This invention relates to a method and apparatus for the identification and processing of bulk metallic glass forming alloys, and the manufacture of components fabricated from these alloys.
2. Description of the Related Art
During the past decade, numerous university and industrial research groups have invested significant efforts to discover metal alloy formulations which form glass when cooled from the molten state at relatively slow rates of cooling. Vitrification or glass formation in metal alloys was first discovered in about 1960 by P. Duwez and coworkers at the California Institute of Technology (xe2x80x9cCaltechxe2x80x9d). They used xe2x80x9crapid solidificationxe2x80x9d techniques to produce glassy alloys by cooling the liquid alloy at rates of about 1 million degrees per second. A variety of such rapidly quenched metallic glasses were produced over the following three decades.
Beginning in about 1990, the research groups of Prof. A. Inoue in Japan and Prof. W. L. Johnson at Caltech developed complex alloys containing 3, 4, 5 or more components which formed glasses at far lower cooling rates, typically 1-100 degrees per second when solidified from the equilibrium liquid state. During the past several years, these xe2x80x9cbulkxe2x80x9d glass forming alloys have attracted substantial commercial interest as engineering materials. The bulk metallic glasses have high strength, high hardness, high specific strength, and a variety of other useful engineering properties. Furthermore, the liquid alloys are highly processable and can be cast into complex three dimensional very near net shapes. Interest in utilizing these materials in engineering applications has led to widespread interest in developing and discovering new bulk glass forming alloys, in processing these alloys from the melt, and utilizing these alloys to produce commercially useful materials such as rods, plates, sheets, tubes, and other more complex shapes.
A key parameter which distinguishes alloys with exceptional glass forming ability is their relatively low melting point. Alloys which form bulk metallic glass undergo equilibrium melting of the initial alloy over a range of temperatures which are relatively low compared with the compositionally averaged melting point of the pure metals which comprise the alloy. Very often, the optimum glass forming alloy lies near a minimum in the melting surface (liquidus surface) of the alloy taken as a function of the alloy composition. This liquidus surface is conventionally represented in alloy phase diagrams as a xe2x80x9cliquidus projection.xe2x80x9d For example, in a two component alloy, the liquidus curve can be represented as a curve in the composition/temperature plane.
A simple example is shown in the Auxe2x80x94Si phase diagram of FIG. 1. This phase diagram of binary Auxe2x80x94Si alloy shows a eutectic composition 10 with a eutectic temperature 12 of 363xc2x0 C. The liquidus line 14 represents the line above which a single liquid phase is present. The solidus line 16 represents the line below which the system has completely solidified. Note that the melting point of Au is about 1064xc2x0 C. and that of Si is about 1414xc2x0 C. Also illustrated is the compositional partitioning of a slowly cooled liquid at an off-eutectic composition 18 during solidification where xL is the composition of the remaining liquid.
In this simple binary case, the eutectic composition is precisely the compositional range where Duwez and coworkers produced the first metallic glass by rapid quenching. More generally, low lying liquidus temperatures (e.g. near alloy eutectic compositions) locate the optimum glass forming regions in higher order ternary, quaternary, quinary, etc. glass forming alloys. See W. L. Johnson, 24 Materials Research Society Bulletin 42-56 (October 1999). Generally speaking, the ability to form metallic glass is optimized at or near eutectic compositions, or more generally near the lowest lying temperatures of the liquidus surface in ternary, quaternary and higher order alloys. As such, the search for easy glass forming alloys is very frequently found to be equivalent to finding those alloy compositions corresponding to the lowest lying melting temperatures (or lowest lying liquidus surface). Most often, the best glass forming alloys lie within about xc2x15 at. % of a minimum in the liquidus surface. The search for easy glass forming alloys is thus dramatically simplified when the lowest melting alloys can be identified.
To locate glass forming compositions, it is critical to be able to identify or xe2x80x9cdiscoverxe2x80x9d alloys with chemical compositions located near the lowest lying regions of the melting curves in the higher order alloys. Thus, it is of interest to develop an efficient means of both discovering and isolating the lowest melting alloys in a complex multicomponent alloys system containing two, preferably three or more metals. In the case of alloys with more than three components, the phase diagrams are generally not available and little or no information exists to guide the researcher to the optimum low melting compositions.
It has actually been proposed to develop materials combinatory methods whereby one searches for low melting alloys by literally making thousands of alloy compositions (using for example thin film processing methods) and screening their melting points in a rapid and parallel manner. See 159 Chem. Week 57 (1997). This approach requires carrying out literally thousands or tens of thousands of screening experiments. Thus, what is needed is a method of identifying the lowest melting alloy in a single or small number of experiments on a bulk liquid.
The production of bulk metallic glass, in addition to developing low melting point alloys, also requires that the low melting liquid alloy be substantially free of contaminants, oxides, and debris which induce crystallization. For metals, the most frequently encountered contamination of the liquid is in the form of crystalline oxide particles, carbide particles, and a variety of other types of nascent foreign substances. These contaminants are ubiquitous and ever present in the processing of nearly all commercial metals used in casting of metallic components. In most common cases (e.g., casting of aluminum, iron, and titanium alloys), they are inevitable inclusions in the liquid. It is well known that this nascent contamination frequently induces crystallization of the liquid alloy when it is undercooled below its melting point. Metallurgists refer to this as heterogeneous crystal nucleation. Practically speaking, heterogeneous nucleation is extremely harmful to the glass forming ability of metal alloys. As such, it would be extremely desirable to develop methods to reduce or eliminate nascent oxide, carbide, and other debris from metallic melts when attempting to produce metallic glass. Thus, what is needed is a direct and efficient means of removing most or nearly all of this nascent contamination and debris from a molten alloy.
Finally, it is of importance to produce bulk metallic glass in useful shapes. What is needed is a natural means of efficiently cooling and solidifying or casting a xe2x80x9cdecontaminatedxe2x80x9d alloy with an optimized composition (having the lowest melting point) into plates, rods, tubes, or other very near net shape castings.
The preferred embodiments of the present invention address these and other needs by providing a rapid and efficient method to identify and physically isolate alloy compositions of low melting alloys which readily form bulk metallic glass. Certain preferred embodiments also process these materials in a manner which removes unwanted and harmful impurities and debris (such as crystalline oxide, carbide or nitride particles) in the molten alloy to improve the glass forming ability of the liquid alloy. Preferred embodiments also describe the production and manufacture of large net shape castings, plates, rods, and other useful shapes from the purified and compositionally optimized liquid alloy.
In one aspect of the present invention, a method of identifying the lowest melting eutectic composition of an alloy having xe2x80x9cnxe2x80x9d phases is provided, where nxe2x89xa72. An arbitrary starting alloy is provided. The alloy is heated until it is substantially molten. Preferably, the molten alloy is subjected to an inertial force above its melting point for a period of time. The temperature of the alloy is lowered while subjecting the alloy to a large inertial force or acceleration. An inertial acceleration is often also referred to as gravitational acceleration or simply gravity where the gravity and acceleration have the opposite sense. The lowering of the temperature causes nucleation and growth of crystals of a first crystalline solid phase within surrounding liquid.
Crystals of the first solid phase are subjected a body or inertial force preferably caused by inertial acceleration or gravity) such that the first solid phase moves either upward or downward (with respect to the direction of the inertial acceleration) in the remaining liquid. As used herein, upward and downward (as well as top and bottom) do not necessarily refer to the ordinary meanings of these terms, but rather indicate direction with respect to the inertial acceleration, where bottom is opposite the direction of acceleration (or along the direction of applied gravity). The direction of motion (i.e., upward or downward) depends on the sign of the density difference between the crystals and the surrounding liquid. Heavier crystals move opposite to the direction of inertial acceleration; lighter crystals move along the direction of the inertial acceleration. This process is commonly referred to as sedimentation.
Further lowering of the temperature of the alloy while subjecting the alloy to said inertial acceleration and body forces causes further nucleation and growth of additional solid phases, the additional solid phase crystals being subjected to the inertial forces such that the additional solid phases move upward or downward, depending on the sign of the difference in density between these crystals and the remaining liquid. Thus, as crystalline phases are sequentially formed, the crystals sediment to the top or bottom of the liquid forming strata at the top and bottom of the vessel which contains the liquid.
The temperature is further lowered until the alloy is substantially completely solidified. The last solid phase to solidify is desirably located between the previously solidified strata. However, it will be appreciated that where the solid phases have substantially the same density, the last solid phase to solidify does not necessarily have to be between other previously solidified strata. The final liquid will have the lowest melting point of all the solidified strata. In the case where this final liquid has a eutectic composition with a minimum melting temperature, the final liquid remaining will solidify at the eutectic composition in a well defined layer or strata. An alloy of eutectic composition will be physically isolated in a well-defined layer.
In another aspect of the present invention, a method of producing a bulk sample of a lowest melting eutectic composition of an alloy is provided. An arbitrary starting alloy is provided. The temperature of the alloy is lowered while subjecting the alloy to an inertial force, the lowering of the temperature causing nucleation and growth of a first solid phase within surrounding liquid. The first solid phase is subjected to the inertial force such that the first solid phase moves upward or downward in the surrounding liquid. Further lowering of the temperature of the alloy while subjecting the alloy to said force causes further nucleation and growth of additional solid phases, the additional solid phases being subjected to the inertial force such that the additional solid phases move upward or downward. The temperature is further lowered until the alloy is substantially completely solidified. A bulk sample of alloy is then removed from the strata or layer which was the last to solidify. This lowest melting sample is then remelted and cast in an effort to produce a bulk glass casting having the composition of this lowest melting alloy.
In another aspect of the present invention, a method of processing a multicomponent alloy, comprising melting the alloy and subsequently solidifying the alloy, both operations being carried out in the presence of a centripetal acceleration field. In one embodiment, the alloy is solidified in the presence of an inertial acceleration or g field of between about 1 and 106 g""s produced by centrifugal motion. Here, g is the acceleration of the Earth""s gravitational field=9.8 m/s2.
In another aspect of the present invention, a method of forming a purified, multicomponent bulk metallic glass forming alloy is provided. A sample alloy is melted at an elevated temperature. The molten alloy is subjected to a centripetal acceleration while holding it above the melting point for a period of time. The alloy is solidified by lowering the temperature of the alloy while continuing to subject the alloy to a centripetal acceleration, the solidified alloy having a portion separated from the remaining alloy having a lowest melting eutectic composition. The portion of the alloy having the lowest melting eutectic composition is isolated. The portion of the alloy having the lowest melting eutectic composition is re-melted at an elevated temperature while subjecting this portion to a centripetal acceleration. The portion of the alloy having the lowest melting eutectic composition is cooled while subjecting the portion to a centripetal field, the cooled alloy having a portion with relatively fewer impurities than the remaining alloy.
In another aspect of the present invention, a metallic alloy having a composition optimized for bulk glass forming ability is provided. The composition is obtained by melting the alloy followed by a slow and gradual cooling and solidification of an initially molten alloy in a centrifuge. The centrifuge subjects the alloy to high inertial forces during solidification which physically separates crystalline phases from the remaining molten alloy, such that the optimized composition of the metallic alloy is the last portion of the alloy to solidify and is physically isolated in a well-defined layer of the final alloy.
In another aspect of the present invention, a high temperature centrifugal processing device for processing molten metal alloys under very high inertial accelerations is provided. A rotor is fabricated of a high temperature material having high strength and fracture resistance at temperatures of between about 400 and 1200xc2x0 C. and which is capable of withstanding inertial accelerations up to at least 50,000 g""s. A plurality of internal cavities within the rotor symmetrically laid out within the body of the rotor. A shaft onto which the rotor is mounted allows the rotor to be spun at high rotation frequencies of between about 1000 and 100,000 rpm.